A torque wrench is a tool which is used to apply a precise amount of torque to a fastener such as a nut or a bolt. Applying a precise amount of torque to a fastener can be important in many situations, e.g., such as when installing or removing the main rotor shaft of a helicopter.
In general, the torque wrench comprises a long lever arm extending between the wrench handle and the wrench head. A torque monitoring system is incorporated in the torque wrench in order to show the operator exactly how much torque is being applied to the fastener. The torque monitoring system is typically incorporated in the long lever arm or in the wrench head.
By way of example but not limitation, in a beam-type torque wrench, the long lever arm is generally made of a material which bends elastically in response to an applied load. By comparing the extent to which the long lever arm deflects (e.g., by comparison to a smaller, non-bending bar also connected to the wrench head), the amount of torque being applied to the fastener can be determined.
Many other types of torque wrenches are well known in the art, some utilizing pressure transducers or strain gauges to measure lever arm deflection or wrench head deformation, and some including mechanical multipliers in the wrench head for amplifying the amount of torque applied to the fastener.
It can be technically challenging to provide a torque wrench having a torque monitoring system which is highly accurate across a wide range of different torque levels. By way of example but not limitation, in many prior art designs, the torque monitoring system provided on a torque wrench might be reliable to + or −3% at low torque levels (e.g., approximately 100 ft-lbs), but only reliable to + or −10% at high torque levels (e.g., approximately 1000 ft-lbs). In this respect it will also be appreciated that higher error ranges at higher torque levels increase the possibility of accidentally over-torqueing a fastener at the higher torque ranges, sometimes with catastrophic results (e.g., fastener breakage, workpiece damage, etc.). Stated another way, if a torque monitoring system is reliable to + or −3% at 100 ft-lbs, the maximum accidental over-torqueing at 100 ft-lbs of torque is only 3 ft-lbs, whereas if a torque monitoring system is reliable to + or −10% at 1000 ft-lbs, the maximum accidental over-torqueing at 1000 ft-lbs of torque is 100 ft-lbs. For this reason, it is generally desirable that the torque monitoring system be as accurate as possible across the full range of torque levels which will be encountered by the torque wrench.
It has also been found that, when using strain gauges and the like to monitor torque levels, the positioning of the strain gauges on the torque wrench can make a large difference in the accuracy of the torque monitoring system, particularly at higher torque levels. This is because various portions of the torque wrench may deform at different rates under different torque loads. Thus, for example, where the torque measuring system uses a strain gauge applied to the cylindrical outer wall of the wrench head to measure applied torque, one level of accuracy may be achieved, and where the torque measuring system uses a strain gauge applied to a flange mounted to the cylindrical outer wall of the wrench head to measure applied torque, another level of accuracy may be achieved. And in either case, this level of accuracy tends to differ significantly across the spectrum of applied torque.
In addition to the foregoing, it has also been found that, with prior art torque wrenches, and particularly with prior art torque wrenches which include mechanical multipliers for amplifying the amount of torque applied to the fastener, some residual forces typically remain on the torque wrench after torque is no longer being applied to the torque wrench. As a result, the torque monitoring system still reports torque on the torque wrench even when no torque is being applied to the torque wrench. It is believed that these residual forces are the result of internal friction, and parts binding, within the torque wrench.
Furthermore, when the application of torque in one direction (e.g., clockwise torque) is replaced by the application of torque in the opposite direction (e.g, counterclockwise torque), the newly-applied torque initially works to nullify the residual opposing torque already stored in the torque wrench. As a result, the torque monitoring system will report that no torque is being applied to the torque wrench, when in fact torque is being applied to the torque wrench. Thus, where the torque wrench stores torque in the torque wrench, there is a “deadband” effect whenever the application of torque in one direction is replaced by the application of torque in another direction. This “deadband” effect essentially undermines the accuracy of the torque monitoring system, since there is a disparity between the level of torque being applied to the torque wrench and the level of torque being reported by the torque monitoring system. Significantly, this disparity is typically non-linear, leading to larger disparities at higher torque levels.
In practice, it is generally necessary, whenever changing the direction of applied torque, to perform a “zero shift” for the torque wrench before applying the opposite torque, in order for the torque monitoring system to accurately register the new torque being applied to the torque wrench. This need to provide a “zero shift” before changing the direction of torque is of significant concern, since the “zero shift” operation is time-consuming and, due to the non-linearity issues discussed above, difficult to apply precisely across a wide range of torque levels. Furthermore, in practice, it has been found that field personnel frequently fail to perform the aforementioned “zero shift” operation, thereby resulting in the torque monitoring system inaccurately reporting the level of torque being applied by the torque wrench.